1. Field of the Invention
The present invention relates to a method for thermal processing a semiconductor wafer.
2. Description of the Related Art
A process for fabricating semiconductor device comprises a step of forming an impurity diffusion layer in the region of the surface layer of the semiconductor wafer, wherein the layer is formed by introducing an impurity by means of ion implantation into a silicon crystal in the region of the semiconductor wafer surface layer, for example, and then thermal processing the semiconductor wafer in this condition at or above 1000° C. for example to activate the impurity.
FIG. 14 is an illustrative cross-sectional view emphasizing the structure of a transistor element formed in a semiconductor wafer. The semiconductor wafer has silicon oxide films (SiO2) 62, 62 embedded in a P type silicon substrate 61 so as to form a region separating from the element thereof, for example, and a gate electrode 66 is formed on the substrate 61, wherein the gate electrode is formed by forming a gate oxide film layer 63 on this silicon substrate 61, and then laminating a tungsten layer 65 on the gate oxide film layer 63 with a polycrystalline silicon layer 64 lying between the gate oxide film layer 63 and the tungsten layer 65. So-called silicon nitride film sidewall spacers 67, 67 are formed on either side of the gate electrode 66. In the figure, 68, 68 indicate ion implantation regions of electrodes, either one of these electrodes is the source and the other is the drain.
In the step of forming the impurity diffusion layer, when the semiconductor wafer to which impurity has been introduced is exposed to an elevated temperature for a long period, the impurity diffuses as far as a region that lies deeper than the region in which the impurity diffusion layer of the semiconductor wafer is to be formed such that the depth (thickness) of the impurity diffusion layer is considerable, and it is therefore necessary to suppress the thermal diffusion of the impurity by ensuring that the semiconductor wafer is not heated beyond what is necessary.
As a method for thermal processing the semiconductor wafer, a technique is known that employs an RTP (Rapid Thermal Process) apparatus that employs a lamp as the heat source, and that is capable of rapidly heating the semiconductor wafer being processed by irradiating the semiconductor wafer with light emitted by this heat source, and then rapidly cooling the semiconductor wafer. Halogen lamps are widely used as this RTP apparatus heat source.
However, in recent years, due to the increasing demand for high integration and increased intricacy of semiconductor integrated circuits, the need has arisen to more shallowly form the impurity diffusion layer in the range of 20 nm or less, for example. Such a demand cannot be adequately met by means of a thermal processing method that employs an RTP apparatus whose heat source is a halogen lamp.
As a method for forming a highly shallow impurity diffusion layer, a technique that employs an apparatus which uses a xenon chloride laser and performs thermal processing by scanning the semiconductor wafer with an irradiation width of a few millimeters using this xenon chloride laser is known. Such an apparatus is currently introduced to semiconductor device fabrication processes at a very sophisticated level but is highly costly and, because thermal processing is carried out while scanning the surface of the semiconductor wafer with a laser beam having a small spot diameter, there is the drawback that the throughput is reduced.
There have therefore been studies into the use of flash discharge lamps, which are capable of heating a product being processed in a very short time, as the heat source of the RTP apparatus. With a thermal processing method that employs an RTP apparatus whose heat source is a flash discharge lamp, flash irradiation of the processed semiconductor wafer by means of the flash discharge lamp, is completed within a very short time, and it is therefore possible to perform heating in a state where heat is transmitted only to the surface layer region and not within the semiconductor wafer, meaning that thermal diffusion of the impurity can be suppressed.
Meanwhile, a thermal processing method is widely adopted in which preheating means for preheating the semiconductor wafer to a predetermined temperature beforehand are provided for example in the RTP apparatus and the semiconductor wafer is heated by the heat source after being preheated by the preheating means.
By preheating the semiconductor wafer, the amount of energy which the heat source requires in order to heat the semiconductor wafer surface layer region to the desired temperature is reduced, and, as a result, the load on the lamp that constitutes the heat source can be reduced and the lifespan of the lamp can therefore be increased.
The preheating temperature produced by preheating means is equal to or less than 550° C. because when the temperature of the semiconductor wafer is equal to or greater than 700° C., the thermal diffusion of the impurity is considerable. In actuality, the preheating temperature is set as high as 450° C. or more for the purpose of reducing the load on the flash discharge lamp, and there are therefore frequent cases of usage in the range 450° C. to 550° C.
However, the problem exists that, in reality, the majority of semiconductor wafers which are thermal processed by being preheated and then irradiated with the flash emitted by the flash discharge lamp are damaged and break up. Further, mass production of semiconductor device fabrication is not possible by means of a thermal processing method that employs a thermal processing apparatus whose heat source is a flash discharge lamp. As a result, it has been difficult to put this thermal processing method into practical use.
As a result of extensive research with the object of putting a thermal processing method that employs a thermal processing apparatus whose heat source is a flash discharge lamp into practical use, the present inventors made the discovery that heating the semiconductor wafer causes compressive stress to be generated in the surface layer that comprises the surface irradiated with the flash from the flash discharge lamp, and in the backside layer that comprises the backside of the semiconductor wafer. However, tensile stress is generated in an intermediate layer that lies between the surface layer and backside layer, and, because the semiconductor wafer possesses the characteristic of low resistance to tensile stress in comparison with compressive stress, the semiconductor wafer is damaged as a result of the tensile stress generated in the intermediate layer.